The size of the cerebral cortex increases during mammalian evolution, underlying an increased complexity of cognitive operations and specialization of functions. Little is known of the mechanisms that control the number of neurons generated in this region. Basic Fibroblast growth factor (FGF2) is expressed by cortical progenitor cells and increases their proliferation both in vitro and in vivo. Adult rats microinjected with FGF2 in the cerebral ventricles during embryogenesis have an increased number of cortical neurons, whereas mice lacking the FGF2 gene product have fewer neurons and glia in the cerebral cortex. The mode of action of FGF2 during corticogenesis is not clear. We hypothesize that FGF2 increases the number of rounds of cell divisions leading to an expansion in the population of cortical progenitor cells, and that FGF1, another FGF ligand, exerts a similar function. In the first specific aim, we test whether FGF2 regulates the number of neurons and glia, whether this action is restricted to the cerebral cortex and what stage of development FGF2 is required. This will be accomplished by morphometric and immunocytochemical analyses at different stages of development of FGF2 null (FGF2 -/-) and FGF2 transgenic mice, which, respectively, lack and over-express the FGF2 gene. We will also examine the phenotype of the FGF2 and other genes. In Specific Aim 2, we study the mechanism of action of FGTF2 on progenitor cells in vivo. We will compare FGF2-/- and FGF2 transgenic mice with regard to volume, cell number and apoptosis within histogenic domains of the telencephalon containing neuronal and glial progenitor cells. In another experiment, the fraction of progenitor cells that re-enters the cell cycle and the fraction that exits the cycle will be measured over the neurogenetic interval within the cortical neuroepithelium. In Specific Aim 2, the role of FGF2 in the regulation of cortical connectivity will be studied. Neurite outgrowth will be measured in neurons grown from FGF2-/- and wild type mice in vitro. To assess the proportion of synapses, dendrites and axons in FGF2-/- and wild type mice in vitro. To assess the proportion of synapses, dendrites and axons in FGF2-/- and wild type mice and in vivo, electron microscopic studies will be carried out. In Specific Aim 4, we will determine FGF1, like FGF2, regulates neuron number in the brain and whether it acts redundantly with FGF2. FGF1 null mutant mice will be compared with the FGF2 null mutants and with FGF1 and FGF2 null compound homozygotes and heterozygotes. Morphometric and immunocytochemical analyses will include areas where FGF1 and FGF2 are jointly expressed during development.